Automotive vehicle engines are generally equipped with fuel injectors for injecting fuel (e.g., gasoline or diesel fuel) into the individual cylinders or intake manifold of the engine. The engine fuel injectors are coupled to a fuel rail which contains high pressure fuel that is delivered by way of a fuel delivery system. In diesel engines, conventional fuel injectors typically employ a valve that is actuated to open and close to control the amount of fluid fuel metered from the fuel rail and injected into the corresponding engine cylinder or intake manifold.
One type of fuel injector that offers precise metering of fuel is the piezoelectric fuel injector. Piezoelectric fuel injectors employ piezoelectric actuators made of a stack of piezoelectric elements arranged mechanically in series for opening and closing an injection valve to meter fuel injected into the engine. Examples of piezoelectric fuel injectors are disclosed in U.S. Pat. Nos. 4,101,076 and 4,635,849. Piezoelectric fuel injectors are well-known for use in automotive engines.
The metering of fuel with a piezoelectric fuel injector is generally achieved by controlling the electrical voltage potential applied to the piezoelectric elements to thereby vary the amount of expansion and contraction of the piezoelectric elements. The amount of expansion and contraction of the piezoelectric elements varies the travel distance of a valve piston and, thus, the amount of fuel that is passed through the fuel injector. Piezoelectric fuel injectors offer the ability to precisely meter a small amount of fuel. However, piezoelectric fuel injectors also generally require relatively high voltages (typically in the hundreds of volts) and high currents (tens of amps) in order to function properly.
Known conventional drive circuitry for controlling a piezoelectric fuel injector is generally complicated and requires extensive energy. This energy is usually provided by a dedicated power supply such as a transformer which steps-up the voltage generated by the vehicle battery (e.g., 12 volts) to a higher voltage (e.g., 230 volts). The step-up voltage is then applied to large reservoir capacitors for powering the charging and discharging of one or more fuel injectors for each injection event. This dedicated power supply generates enough energy to maintain the reservoir capacitor voltage over the full operating load and speed range of the engine. However, a disadvantage of providing a dedicated power supply of this size is increased cost. Thus, a further disadvantage is that the controller required to control the drive circuit must be of large size.
German patent application no. DE 102 45 135 A1 (Nippon Soken, Inc. et al) describes a drive circuit for controlling a plurality of piezoelectric fuel injectors. The drive circuit comprises a DC/DC voltage converter 21 for stepping up the voltage produced by a vehicle battery. The stepped-up voltage is applied to capacitors in the circuit which are then used for charging the fuel injector piezoelectric elements. The drive circuit comprises a single voltage supply rail and operates in a purely unidirectional manner (i.e., it does not provide negative voltages), and therefore cannot be used to drive bi-directional fuel injectors which require both negative and positive voltages.
It has been suggested that vehicle manufacturers are planning to replace 12 volt vehicle batteries with a 42 volt charging system. This change has been prompted by the move to replace mechanical and hydraulic systems with electronics (i.e. “drive-by-wire”), and will provide a way to improve fuel economy and reduce emissions. Another problem with current drive circuits is that it is difficult to control dynamically the voltage across the large reservoir capacitors should the higher 42 voltage supply (or a lower voltage supply) be required.
An object of the invention is therefore to provide a drive circuit which requires less components than existing drive circuits for injector arrangements, and which is therefore cheaper and more controllable than such drive circuits. Another object of the invention is to provide a drive circuit which is suitable for use with voltage supplies having different capabilities.